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An Introduction to Tape/MIDI Sync and the MQX-32
Dave Hocker
Copyright (c) 1988 - All Rights Reserved
Music Quest, Inc.
Overview
Since the introduction of the Music Quest MQX-32 Enhanced MIDI Co-processor
Card, we here at Music Quest have fielded a large number of questions about
the features of the MQX-32 and the various different forms of tape/MIDI
sync. The purpose of this paper is to discuss the most common forms of
sync in use by computer/music/MIDI systems, with particular emphasis on
tape sync. In doing so, we will look at how each sync method is
implemented. Also, we will consider the relative strengths and weaknesses.
After you complete reading this paper, you will have a basic understanding
of these different sync methods, and you will have a good perspective on
the sync features of the Music Quest MQX-32 PC MIDI interface. Hopefully,
we will have answered many of your questions, while simultaneously
providing enough information so you can ask new ones.
Types of Sync
We're going to discuss four different types of sync.
MIDI sync
FSK (Frequency Shift Keying)
CLS (Chase Lock Sync)
SMPTE (Society of Motion Picture and Television Engineers)
In our discussions, we will treat MIDI sync differently than the other
three methods, mainly because it is not a tape sync technique, per se.
Near the end of this paper we will discuss various converters that can
convert from one sync source to/from another.
Some Basic Concepts
Before getting into the details of sync methods, we need to get a few
basics under our belt. First let's make sure we all understand the general
reason for sync. The term sync is used to refer to various techniques that
allow multiple, time dependent devices to work together. For MIDIists,
this usually means that you want to sync multiple MIDI devices together or
that you want to sync a tape recorder with one or more MIDI devices. The
second reason is probably the most common circumstance, as it is used to
overcome a shortage of sound generators or synthesizers. It allows you to
create a finished performance by reusing sound generators on separate
tracks. That is, you make several passes on the tape, recording a new
track on each pass.
When you are synchronizing, you always have one master and one or more
slaves. A good analogy is a conductor and an orchestra. The conductor is
the master sync source who keeps all of the orchestra's musicians together.
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There is another sync basic that we can draw from the conductor analogy,
One of the roles of the conductor is to set the tempo for the orchestra.
The conductor uses his/her baton as a "clock". The faster the baton moves,
the faster the tempo, and vice versa.
Another concept we need to understand is the difference between sync
methods that are "tempo relative" and ones that are "fixed time". With a
tempo relative sync method, the rate of the sync signals is directly
proportional to the tempo. That is, the sync signals are generated at a
rate that is determined by the beats-per-minute tempo. As we shall see,
MIDI sync, FSK, and CLS are tempo relative sync methods (our orchestra
conductor is also a tempo relative sync source).
A fixed time sync method uses signals that are not proportional to tempo.
The sync signals are generated at a fixed rate. Of the four sync
techniques we will discuss, SMPTE is the only one that uses fixed timing.
When we are synchronizing a tape recorder and MIDI devices, we use one of
the tape tracks to store the sync information. The process of "striping a
tape" refers to the task of writing the sync information on the sync track.
After the tape is striped, it becomes the master source of sync signals for
the system.
MIDI Sync
MIDI sync is most commonly used to allow one or more "slave" MIDI devices
to be synchronized to a "master" MIDI device. Typically, these devices are
sequencers or devices with "sequencer like" capability (e.g. drum
machines). MIDI sync uses a combination of MIDI messages to accomplish its
goal.
????????????????Out In ?????????????????????
? ????????????????????????>? ?
? Sequencer ? MIDI Sync -> ? Drum ?
? ? ? Machine ?
? ? ? ?
???????????????? ?????????????????????
The heart beat of MIDI sync is the MIDI clock message (simply known as the
MIDI clock). The MIDI master sends MIDI clocks at a rate of 24 clocks per
quarter note. Hence, MIDI sync is tempo relative. MIDI devices need to
know when to play and when to stop playing. The MIDI Start, Continue, and
Stop messages provides these signals. When a MIDI device receives a MIDI
Start, it plays from the beginning of its current song. When it receives a
MIDI Stop, it ceases play. When a MIDI Continue is received, the MIDI
device plays from the current position within the current song.
This brings us to the MIDI Song Position Pointer (SPP) and MIDI Song Select
(SS) messages. Simply, the SS message tells the device which song to play
from, and the SPP tells the device where to start within that song. In
practical application, only the SPP is used. Now, if you put the SPP
together with a MIDI Continue, you have a most useful way to synchronize
MIDI devices. This combination of messages essentially says "Go to this
position within the current song and begin playing". Most often this
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technique is referred to as "chase sync". Later, we will see how this
principle is applied to CLS tape sync.
[SPP]...[Continue] [Clock]...[Clock]...[Clock]...[Clock].....
FSK Sync
FSK is most commonly used to synchronize a tape recorder with MIDI devices,
such as a sequencer or drum machine. Typically, FSK uses two distinctly
different tones to form a sync signal (e.g 1200 Hz and 2400 Hz). One tone
is used to fill the tape, while the second is used to mark the occurrence
of a clock signal. As the tape is read, the sync device generates a clock
signal every time it sees the transition from fill signal to mark signal.
Hence, the two tones form a binary tape encoding.
FSK sync can be found on many of the MPU-type PC MIDI interfaces. Some
stand alone sequencers and drum machines are also capable of the writing
and reading FSK.
One of the advantages of FSK is that it can be written at various degrees
of clock resolution. For example, with the MPU-type FSK, you can write a
sync track at MIDI clock resolution (24 ticks per beat) or at the same
resolution as the MPU's internal clock (up to 192 ticks per beat).
In practical application, FSK sync has several draw backs. Since it is a
purely binary signal containing no position information, you always have to
start at the beginning of the sync track. FSK sync has no counterpart to
the MIDI Song Position Pointer. FSK by its very nature is frequency
sensitive, and its reliability can be greatly affected by tape quality,
tape speed, and volume level. Finally, since FSK contains no position
information, error recovery from tape drop outs is very difficult.
Chase Lock Sync
Quite simply, CLS can be described as the concept of MIDI sync applied to
tape. That means that CLS is a tempo relative sync method. Unlike FSK,
CLS does not use tones. Instead it uses wave forms much like those used by
computer tape drives. The sync information is written onto the tape as a
series of bits. Included in the sync information are song position
pointers and MIDI clocks. Thus, when the sync track is read back, it is
possible to locate exactly where the tape is positioned (starting from any
place on the sync track). Usually, the sync device reads the position
information, forms it into a MIDI song position pointer message, and sends
it to the sequencer. The sequencer responds to the SPP by "chasing" the
current sequence to the location indicated by the SPP (hence the term Chase
Lock Sync). After allowing some time for the sequencer to "chase", the
sync device sends a MIDI continue which initiates sequencing.
CLS has one major short coming, namely its clock resolution of 24 ticks per
beat. However, this draw back is greatly over shadowed by CLS's chase
capability. When you use CLS with the appropriate sequencer, you can play
and record from any location on the tape. Unlike FSK, you do not have to
start at the beginning of the tape. Another plus for CLS is its encoding
technique which facilitates excellent recovery from tape drop outs.
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In summary, CLS is a significant step forward from FSK.
SMPTE
Of all the three tape sync methods being discussed, SMPTE is the only one
for which a true standard exists (of course, MIDI is also a true standard).
It is also the only fixed time sync method. Originally, SMPTE was
developed for motion picture and television work. However, as we shall
see, its utility has widened its application.
SMPTE uses electrical signals that are similar to CLS. That is, the sync
information is written as a string of bits. However, SMPTE uses the
concept of a "frame" instead of MIDI concepts such as song position pointer
and MIDI clocks. A SMPTE frame is a group of bits that describe a single
location on the tape (called the frame address). The number of bits that
make up the frame, the rate at which the bits are written to tape, and the
number of frames per second are dependent on the SMPTE format. In the US,
the two 30 frame formats (30 frames per second) are used most of the time.
These formats use a frame that is 80 bits long, written at a rate of 2400
bits per second. Each frame address contains a complete time stamp in
terms of hours, minutes, seconds, and frame number. Most often, you see
this written in the form:
HH:MM:SS:FR
Thus 01:37:22:19 identifies the frame address as frame 19, at 1 hour, 37
minutes, 22 seconds. (If you are completely new to this topic and you
watch MTV, you can occasionally see videos with SMPTE consoles showing a
rolling frame address. The Cheap Trick video "The Flame" is a good
example).
Overall, there are four standard SMPTE formats:
24 frame used by motion pictures.
25 frame used in Europe for video work because the
television scan rate is 25 frames per second.
30 frame non-drop format used world wide for audio sync.
30 frame drop format used in the US for video work. US color
television uses a scan rate 29.97 frames per
second. The term "drop" format comes from the
fact that this format is identical to 30 frame
non-drop, except that selected frames are
periodically dropped to yield an overall scan
rate of 29.97 frames/second.
As previously mentioned, in the US, the 30 frame formats are most
prevalent. The 30 frame non-drop format is predominantly used for audio
sync. The 30 frame drop format is used for video/audio sync work, such as
scoring a video tape.
Practically speaking, SMPTE offers more advantages than any other form a
synchronization. This may not be immediately obvious, since we haven't
said anything about how SMPTE can be used. However, it is clear that SMPTE
is wave of the future.
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Several aspects of SMPTE give it a distinct advantage over all of the other
forms of tape sync. First, SMPTE is a standard that has been in use for
some time. A tape written by a SMPTE writer can be read by any SMPTE
reader that supports the given format. For example, if you write a tape in
30 frame non-drop format, then any reader capable of reading 30 frame non-
drop format can read the tape. While MIDI sync is a standard, FSK and CLS
are not. Every manufacturer's FSK and CLS formats are proprietary
(although Roland's FSK sync has been used by other manufacturers), and
hence interchange of tapes is generally not practical.
As further evidence of the value of this standard, SMPTE has been adopted
into the world of MIDI through the MIDI Time Code extension. With out
going into too much detail, MIDI Time Code messages allow SMPTE frame
addresses to be sent over MIDI cables, in real time. Thus, there is
already a tight binding between SMPTE and MIDI. We'll talk more about
SMPTE and MIDI when we look at SMPTE to MIDI Time Code converters.
Second, SMPTE's fixed time format gives it more flexibility than FSK or CLS
tape sync. If you stripe a tape with FSK or CLS, and then decide you want
a different tempo or set of tempos, you must rewrite the tape. With SMPTE
and the appropriate sequencer, you can change tempos at any time without
the necessity of rewriting the sync track.
Third, like CLS tape sync, you can start reading a SMPTE track at any
location. SMPTE's encoding scheme also greatly facilitates recovery from
tape drop outs.
Finally, SMPTE can be used as a trigger mechanism with power and
simplicity. For example, using SMPTE it is easy to visualize starting a
sequence at location 00:03:02:00 (i.e. 3 minutes, 2 seconds into the tape).
It is just as easy to visualize that at that location on a video is a car
crash and that you want your sequencer to fire a set of notes to a sampler
which in turn will play the sound of the crash.
With all of these advantages it is difficult to see any short comings with
SMPTE. Probably the only draw back is the current shortage of SMPTE
applications for the IBM PC world. However, with the introduction of the
Music Quest MQX-32 PC MIDI interface and the Yamaha C-1 music computer
(which both have SMPTE), many developers are quickly moving to fill this
void.
Sync Converters
A sync converter is an external "box" that translates one form of sync into
another. For the most part, sync converters are used to translate a tape
sync format into some form of sync that is recognized by MIDI (either MIDI
sync or MIDI Time Code).
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FSK to MIDI Sync
An FSK to MIDI sync converter can produce FSK tape sync from MIDI sync
messages (the striping process). Conversely, as it reads an FSK sync
track, it produces MIDI sync. The Yamaha YMC-10 is an example of an FSK to
MIDI sync converter.
????????????????Out In ?????????????????????
? ????????????????????????>? ?
? MIDI ? ? FSK to MIDI Sync ?
? Device ? <- MIDI Sync -> ? Converter ?
? ?<???????????????????????? ?
????????????????In Out?????????????????????
In? ^ ?Out
? FSK ?
Out? v ?In
?????????????????
? Tape ?
? Deck ?
?????????????????
Chase Lock Sync
There are several Chase Lock Sync devices on the market, such as the J.L.
Cooper PPS-1, Synhance MTS-1, and Tascam MTS-30. The Music Quest MQX-32 PC
MIDI interface features built in CLS.
CLS boxes perform just like FSK to MIDI sync converters. Their distinct
advantage lies with the advantages of CLS, namely the ability to use song
position pointer to play from any tape location.
????????????????Out In ?????????????????????
? ????????????????????????>? ?
? MIDI ? ? CLS Sync ?
? Device ? <- MIDI Sync -> ? Box ?
? ?<???????????????????????? ?
????????????????In Out?????????????????????
In? ^ ?Out
? CLS ?
Out? v ?In
?????????????????
? Tape ?
? Deck ?
?????????????????
SMPTE to MIDI Sync
SMPTE to MIDI sync converters provide capabilities similar to CLS sync
converters. However, they offer distinct advantages by virtue of their use
of SMPTE as the tape sync method.
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???????????????? ?????????????????????
? ? ? ?
? MIDI ? ? SMPTE to MIDI Sync?
? Device ? <- MIDI Sync ? Converter ?
? ?<???????????????????????? ?
????????????????In Out?????????????????????
In? ^ ?Out
? SMPTE ?
Out? v ?In
?????????????????
? Tape ?
? Deck ?
?????????????????
Because SMPTE is a fixed time sync method, it does not require an input
signal to stripe a tape. Thus, SMPTE to MIDI sync converters do not
require MIDI sync input during the striping process.
The playback process is somewhat more complicated; however, it is
correspondingly more flexible. Most SMPTE to MIDI converters require that
you give them sort of a tempo map. There are several ways that this can be
accomplished. One way is to tell the converter what MIDI sync tempos
should be in effect at what SMPTE frame addresses. For example at frame
00:00:00:00, you might want to start with a tempo of 120 beats/minute.
Then, at frame 00:01:00:00 (one minute into the sync track), you might want
the tempo to change to 140 beats/minute.
A second way to enter a tempo map is to tell the converter box pertinent
operating parameters so that the converter can determine the appropriate
tempo. For example, you could tell the converter that you want to play
measure 1 starting at 00:00:00:00 and that you want measure 30 to finish at
00:01:00:00. Assuming 4 beats/measure, the converter would calculate that
a tempo of 120 beats/minute would be the appropriate tempo.
The necessity of entering a tempo map into an external converter box
illustrates one of the potential draw backs of such a device. If the MIDI
software you use is not capable of automatically loading the tempo map
information into the converter box or if the converter box is not capable
of being totally set up via MIDI, you may find that you have to do it
manually.
Two examples of SMPTE to MIDI converters are the Roland SBX-80 and Imagine
Computer's SMPTE City. The Music Quest MQX-32 PC MIDI interface in
combination with the appropriate software can perform the same functions as
any of these products, including the management of the tempo map.
SMPTE to MIDI Time Code
A SMPTE to MTC converter is capable of reading and writing SMPTE tape sync
and translating the frame addresses into MIDI Time Code messages. Because
MIDI Time Code is relatively new, not too many products of this nature are
available. The value of such a product is to broadcast the equivalent of
SMPTE sync over a MIDI network.
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???????????????? ?????????????????????
? ? ? ?
? MIDI ? ? SMPTE to MTC ?
? Device ? <- MIDI Time Code ? Converter ?
? ?<???????????????????????? ?
????????????????In Out?????????????????????
In? ^ ?Out
? SMPTE ?
Out? v ?In
?????????????????
? Tape ?
? Deck ?
?????????????????
The use of MTC converters in the IBM PC world has been limited until
recently. This is mostly because widely extant MPU chip set based
interfaces do not recognize MIDI Time Code messages. As a result, most IBM
PC software developers have not been able to take advantage of SMPTE to MTC
converters. However, the Music Quest MIDI Co-processor Card and the MQX-32
Enhanced MIDI Co-processor Card are fully capable of handling MTC messages.
The J.L. Cooper PPS-1 is an example of a converter that is capable of
producing MIDI Time Code while reading SMPTE. The Music Quest MCC is
capable of receiving MTC messages from a SMPTE to MTC converter. The Music
Quest MQX-32 PC MIDI interface can produce MIDI Time Code while writing and
reading SMPTE, and it is capable of receiving MIDI Time Code from an
external MTC source (such as a PPS-1 or another MQX-32).
The Future for Tape Sync
As we have seen, tape sync has gone from the basic capability of FSK sync
to the power of CLS and SMPTE. The popularity of SMPTE is on the rise, and
it appears that over the next year it will become THE tape sync method.
The introduction of inexpensively priced SMPTE capable interfaces to the
broad based world of the IBM PC should further speed this movement. Look
for a wide variety of new SMPTE based products to be introduced into the PC
world over the next 12-18 months, as software developers rush to fill the
current void.
You can expect to see mainstream software companies such as Twelve Tone
Systems, Magnetic Music, and Voyetra introduce new or enhanced products
based on their current sequencer product line. All of these products will
exploit SMPTE to some degree. It is likely that you will see the
introduction of totally new products for use with SMPTE. A product based
on the concepts of Digidesign's Cue Sheet is almost certain to appear.
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The Music Quest MQX-32 PC MIDI Interface
The MQX-32 is the latest addition to the Music Quest PC MIDI interface
product line. As the preceding discussion has illustrated, it provides a
significant set of sync features. Furthermore, the MQX-32 is upward
compatible with the MIDI Co-processor Card, as well as interfaces based on
the Roland MPU chip set.
MQX-32 Features
The MQX-32 incorporates many new features not found in current PC
interfaces.
Two separate MIDI-out ports allowing PC software to support two separate
MIDI paths or a total of 32 MIDI channels.
Chase Lock Tape Sync. The MQX-32 can "stripe" a tape so that it can be
started from any location. When the tape is played, the MQX-32
generates a Song Position Pointer which allows a PC sequencer to "chase"
to the current tape location. Unlike ordinary FSK sync, CLS allows you
to start from any place on the tape.
SMPTE. The MQX-32 can write and read 30 frame drop and non-drop SMPTE
format. The MQX-32 is capable of generating MIDI Time Code during SMPTE
reading, so that outboard MIDI devices can respond to the MIDI Time Code
messages. SMPTE support includes a "cue point" facility which
simplifies the development of PC based SMPTE software.
MIDI Time Code support. The MQX-32 can receive MIDI Time Code messages
from MIDI-in. MIDI Time Code and SMPTE are implemented so that PC
software sees them as identical sync sources (i.e. the "cue point"
facility works with both).
Software Support
All software that runs on the current MIDI Co-processor Card will run on
the MQX-32. All existing PC sequencers that support Song Position Pointer
based MIDI sync will work with the MQX-32's Chase Lock Sync. This
includes:
Cakewalk
Forte
Sequencer Plus
The following companies have made commitments to support the MQX-32's new
features:
Twelve Tone Systems (Cakewalk)
Magnetic Music (Texture)
LTA Productions (Forte)
Imagine Group (Master Series)
Club MIDI (ProLib)
The MIDI Connection (Tape 'n' Step).
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Trademarks
All trademarks are the property of their respective companies.
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